Recently, industries related to the application of solar energy have been rapidly flourishing. The application of solar energy usually refers to transforming solar energy into electrical energy or thermal energy for use. Taking the transformation from solar energy to thermal energy as an example, solar thermal collectors are used to absorb solar radiation energy and transfer it into thermal energy. The thermal energy from sunlight is then utilized to heat water or other heat transfer materials for a variety of heating purposes.
At present, various types of solar thermal collectors are available, a part of which uses an absorbing plate to absorb the solar radiation energy, transfer it into thermal energy, and transmit the thermal energy to heat conducting tubes or evaporators, which are attached to the absorbing plate, so as to heat the heat transfer fluid (such as water, thermal oil, or other suitable liquids) within the heat conducting tubes or evaporators. Because the solar thermal collectors of this type are usually exposed to atmospheric pressure during operation, the high heat loss rate decreases the system efficiency and limits the operating temperature. Some other types of solar thermal collectors use a cylindrical absorber placed in a transparent cylindrical container. The cylindrical container encloses a vacuum space between the inner surface of cylindrical container and the outer surface of the cylindrical absorber. The cylindrical absorber absorbs the solar radiation energy so as to heat the heat transfer material within the cylindrical absorber. However, the solar thermal collectors of this type usually require use of glass material to transmit the thermal energy. Consequently, the poor heat conductivity of the glass material restricts the operating temperature and the system efficiency. In addition, some other types of solar thermal collectors allocate one or more absorbing plates in a sealed light-transmissive container. The container may be in a cylindrical or box shape and encloses a vacuum space inside for the absorbing plates to absorb the solar radiation energy in the vacuum state to minimize conduction and convection heat loss. Nevertheless, the solar thermal collectors of this type usually need to be fabricated by glass-metal sealing, which increases the production costs and has a high failure rate during the product life cycle. In addition, the permanently sealed collectors allow no air circulation to release excessive heat due to stagnation or excessive sunlight that may cause collector overheating or system damage.